Toward Development of a Long Rupture History of the Northern Imperial Fault in Mesquite Basin, Imperial Valley, Southern California
Advisor Dr. Thomas Rockwell
The Imperial fault is a 70-km long, northwest-striking, dextral fault located in the Salton Trough, which along with the San Jacinto and Elsinore faults, are considered part of the San Andreas fault system and accommodate a significant proportion of the North American-Pacific plate-boundary motion in southern California. The Imperial fault was not recognized until the 1940 M¬L 7.1 earthquake that was caused by an end-to-end rupture propagating mostly to the southeast. Thirty-nine years later, surface rupture recurred on the fault with a ML 6.6 earthquake. The rupture nucleated south of the International Border and propagated unilaterally to the northwest. We conducted a paleoseismic study at the Dogwood site in the Mesquite Basin along the northern Imperial fault by opening 12 new trenches to extend the record of late Holocene surface ruptures. Our primary goals are to better understand the behavior of this important plate boundary fault and to improve earthquake forecast models. Our trenches exposed evidence for up to 17 events in the past 1200-1400 years. Fourteen of the 17 events are considered as likely events and the other three as unlikely, based on our rating of evidence for events. By calculating the recurrence interval using all 17 events, the northern Imperial fault is estimated to rupture every 75 to 88 years (average 82 years). Excluding the three unlikely events (Events 8.5, 8.7 and 9) in the calculation, the recurrence interval is increased to 92 to 108 years (average 100 years). Regardless of the time frame considered and the inclusion or exclusion of the unlikely events, we conclude that the northern Imperial fault has an average recurrence interval of about 91 years. In addition, there are three hiatuses that range from 175 to 220 years, separating clusters of 5 to 9 events. If this pattern persists into the future, we would expect to have an additional 3 to 7 events occurring on the northern Imperial fault in the next 220 to 360 years, assuming that we have entered into a new earthquake cycle with the 1940 and 1979 earthquakes. Furthermore, based on the average slip rate at this site (~11 mm/year) that Meltzner and Rockwell (2011 in review) determined, we estimate that the average slip per event at the Mesquite Basin along the northern Imperial fault to be approximately a meter.